Process and apparatus for generating a pressurized product by low-temperature air fractionation

ABSTRACT

The process serves for generating a pressurized product by low-temperature air fractionation. Feed air ( 1 ) is compressed ( 2 ), purified ( 4 ), cooled ( 9 ) and fed to a distillation column system ( 12 ) for nitrogen-oxygen separation ( 11, 23 ). A liquid product stream ( 13 ) is taken off from the distillation column system ( 12 ) for nitrogen-oxygen separation, brought ( 14 ) in the liquid state to an elevated pressure (PIV) and at this elevated pressure (PIV) vaporized or pseudovaporized ( 9 ). The (pseudo)vaporized product stream ( 16 ) is fed ( 17 ) as pressurized product to a gas pressure reservoir ( 19 ) which has a variable pressure (PA). The elevated pressure (PIV) is varied. The elevated pressure (PIV) is varied as a function of the pressure (PA) of the gas pressure reservoir ( 19 ).

The invention relates to a process for generating a pressurized productby low-temperature air fractionation by means of internal compression inwhich:

-   -   feed air is compressed, purified, cooled and fed to a        distillation column system for nitrogen-oxygen separation;    -   a liquid product stream is taken off from the distillation        column system for nitrogen-oxygen separation, brought in the        liquid state to an elevated pressure and at this elevated        pressure it is vaporized or pseudovaporized and    -   the vaporized or pseudovaporized product stream is fed as        pressurized product to a gas pressure reservoir which has a        variable pressure.

Process and devices for low-temperature fractionation of air aredisclosed, for example, by Hausen/Linde, Tieftemperaturtechnik[Low-temperature technology], 2nd edition 1985, chapter 4 (pages 281 to337). A “distillation column system” comprises at least one separationcolumn and also the condensers and evaporators assigned to theseparation columns of the system. The distillation column system fornitrogen-oxygen separation of the invention can be constructed as aone-column system for nitrogen-oxygen separation, as a two-column system(for example as a classic Linde-twin-column system), or else asthree-column or multiple column systems. In addition to the columns fornitrogen-oxygen separation, it can have other devices for producingother air components, in particular noble gases, for example argonproduction.

In an internal compression process, at least one of the products istaken off in the liquid state from one of the columns of thedistillation column system or from a condenser connected to one of thesecolumns, brought in the liquid state to an elevated pressure, vaporizedor (at supercritical pressure) pseudovaporized in indirect heatexchange, for example with feed air or nitrogen, and finally obtained asgaseous pressurized product and fed to a take-off system which consists,for example, of a gas pressure reservoir. The pressure increase in theliquid can be carried out by any known measure. Generally pumps are usedin this process. However, it is also possible to utilize a hydrostaticpotential and/or the pressure build up vaporization in a tank.

Such internal compression processes are disclosed, for example, by DE830805, DE 901542 (=U.S. Pat. No. 2,712,738/U.S. Pat. No. 2,784,572), DE952908, DE 1103363 (=U.S. Pat. No. 3,083,544), DE 1112997 (=U.S. Pat.No. 3,214,925), DE 1124529, DE 1117616 (=U.S. Pat. No. 3,280,574), DE1226616 (=U.S. Pat. No. 3,216,206), DE 1229561 (=U.S. Pat. No.3,222,878), DE 1199293, DE 1187248 (=U.S. Pat. No. 3,371,496), DE1235347, DE 1258882 (=U.S. Pat. No. 3,426,543), DE 1263037 (=U.S. Pat.No. 3,401,531), DE 1501722 (=U.S. Pat. No. 3,416,323), DE 1501723 (=U.S.Pat. No. 3,500,651), DE 2535132 (=U.S. Pat. No. 4,279,631), DE 2646690,EP 93448 B1 (=U.S. Pat. No. 4,555,256), EP 384483 B1 (=U.S. Pat. No.5,036,672), EP 505812 B1 (=U.S. Pat. No. 5,263,328), EP 716280 B1 (=U.S.Pat. No. 5,644,934), EP 842385 B1 (=U.S. Pat. No. 5,953,937), EP 758733B1 (=U.S. Pat. No. 5,845,517), EP 895045 B1 (=U.S. Pat. No. 6,038,885),DE 19803437 A1, EP 949471 B1 (=U.S. Pat. No. 6,185,960 B1), EP 955509 A1(=U.S. Pat. No. 6,196,022 B1), EP 1031804 A1 (=U.S. Pat. No. 6,314,755),DE 19909744 A1, EP 1067345 A1 (=U.S. Pat. No. 6,336,345), EP 1074805 A1(=U.S. Pat. No. 6,332,337), DE 19954593 A1, EP 1134525 A1 (=U.S. Pat.No. 6,477,860), DE 10013073 A1, EP 1139046 A1, EP 1146301 A1, EP 1150082A1, EP 1213552 A1, DE 10115258 A1, EP 1284404 A1 (=US 2003051504 A1), EP1308680 A1 (=U.S. Pat. No. 6,612,129 B2), DE 10213212 A1, DE 10213211A1, EP 1357342 A1, DE 10238282 A1, DE 10302389 A1, DE 10334559 A1, DE10334560 A1, DE 10332863 A1, EP 1544559 A1, EP 1585926 A1, or DE102005029274 A1.

“Gas pressure reservoir” is here taken to mean any system which servesfor buffering gaseous pressurized product and in particular has abuffering capacity which is sufficient to compensate for periodictake-off fluctuations or which is sufficient to compensate for temporarydeficits or surpluses in production which occur during load changes. Oneexample of periodic take-off fluctuations is the oxygen supply to asteelworks in which owing to the operation of the converters at regularintervals, high volumes of oxygen are required in the short-term. Afurther example is an air fractionation unit whose production iscontinuously adjusted to a current consumption, but the load (productionrate) of the air fractionation unit cannot be changed at the same rateas the consumption and therefore temporary deficits or surpluses occurduring the load adjustment. Generally, the buffering capacity of the gasstorage reservoir should be sufficient to compensate for the deficits orsurpluses in production occurring due to a typical change of theconsumption (within minutes or seconds) in such a manner that theproduction of an air fractionation plant can follow the change inconsumption, without the minimum or maximum permitted pressure limits ofthe product being infringed. The load adjustment time of a typical airfractionation unit for a change in load over the full load range of 70%to 100% is 30 minutes to 2 hours.

Since a “gas pressure reservoir” is associated with high capital costs,it will generally not be designed for all possible cases, but only forthe take-off fluctuations typical during normal operation. Exceptionalsituations must be covered if appropriate by blowing off the product orby an additional supply (for example evaporator for cryogenic liquids).

“Gas pressure reservoir” is taken to mean in particular a system whichhas a buffering capacity which is at least equal to the amount of liquidproduct stream (pseudo)vaporizing to the pressurized product which thedistillation column system generated in standard operation within acertain time period, for example at least equal to the amount generatedwithin one minute, in particular at least equal to the amount generatedwithin five minutes, or at least equal to the amount generated within 10minutes. The buffering capacity of a gas pressure reservoir isdetermined by its volume and the possible width of variation of itspressure, that is to say the difference between the maximum and minimumoperating pressure. The minimum operating pressure is established by thepressure requirements of the consumers, the maximum operating pressureby the construction of the gas pressure reservoir and safety regulationsapplicable thereto. A “gas pressure reservoir” can be formed, forexample, by one or more dedicated gas pressure reservoir vessels or by apipeline system having long piping lengths which serves, for example,for supplying a plurality of consumers with pressurized gas. Such a “gaspressure reservoir” is operated in a defined pressure range which isdetermined by a minimum permissible pressure and a maximum permissiblepressure. Between these two values there is typically a difference of atleast 2 bar, in particular at least 5 bar, preferably at least 10 bar.The larger the permissible width of variation of the pressure, thegreater the available capacity in the pressure buffer of the gaspressure reservoir. The necessary capacity of the pressure bufferdepends essentially on the course of the take-off fluctuations which aregenerally subject to a defined systematic change. In order to be able toflow into the gas pressure reservoir, the pressurized product obtainedin the distillation column system must have a pressure which is higherthan the pressure in the gas pressure reservoir. Hitherto this demandwas met by the internal compression product being vaporized at apressure which ensures introduction of the pressurized product into thegas pressure reservoir even at the maximum pressure of the gas pressurereservoir. The pressure during vaporization and also the operatingpressures in the distillation column system are kept constant. In thecase of a currently lower pressure in the gas pressure reservoir, thegaseous pressurized product is throttled, as a result of which energy islost.

SUMMARY OF THE INVENTION

Thus, one aspect of the present invention is to provide a process of thetype mentioned above which operates particularly expediently withrespect to energy.

Upon further study of the specification and appended claims, furtherobjects, aspects and advantages of this invention will become apparentto those skilled in the art.

In accordance with the invention, the elevated pressure (that is to saythe pressure of the internal compression product) is varied and theelevated pressure (PIV) is varied as a function of the pressure (PA) ofthe gas pressure reservoir.

By adjusting the pressure of the internal compression product, thevaporization can take place at reduced pressure when the pressure in thegas pressure reservoir is below its maximum value. This means that lessenergy need be used for vaporizing the product stream.

In an internal compression process, generally a gaseous heat carrierstream is compressed to a high pressure (PW) and used at this highpressure for the (pseudo)-vaporization of the liquid product stream byindirect heat exchange. In the context of the invention it is expedientwhen in this case the high pressure (PW) and/or rate (MW) of the heatcarrier stream is varied and the high pressure (PW) and/or rate (MW) isvaried as a function of the pressure (PA) of the gas pressure reservoir.As a result, in the compression of the heat carrier stream, energy issaved when the pressure of the gas pressure reservoir is below itsmaximum value. In practice, the last-mentioned variation can be directedaccording to the pressure of the internal compression product (PIV); thesaid dependence on the pressure (PA) of the gas pressure reservoir isthen an indirect one.

The heat carrier stream can be formed, for example, by a substream ofthe feed air or by a nitrogen stream from the distillation columnsystem. Frequently, a substream of the feed air is recompressed, used asheat carrier stream, and subsequently introduced into the distillationcolumn system for nitrogen-oxygen separation. “Rate” is taken to meanhere the molar amount per unit time which is measured, for example, inNm³ /h.

In addition, or alternatively, in the context of the invention, energycan also be saved as a result of the fact that the cold generation atreduced pressure (PA) in the gas pressure reservoir is decreased byvarying the amount of cold generated in the cold generation system ofthe process as a function of the pressure (PA) of the gas pressurereservoir.

The cold generation system can comprise one or more expansion machinesfor work-producing expansion of one or more process streams, one or morecold systems driven by external energy and or the supply of cold by oneof more low-temperature liquid streams. Typically, in the invention, therate of one or more process streams passed via an expansion turbine iscontrolled. At reduced pressure in the gas pressure reservoir, this isdecreased. Correspondingly decreased demand for pressure energy leads tofurther energy savings.

In a further embodiment of the process according to the invention, oneor more operating parameters of the distillation column system is variedas a function of the pressure (PA) of the gas pressure reservoir.

It is known to adjust the operating parameters of an air fractionationsystem to variable product rates via a load change system. Such a loadchange system can comprise feed-forward control, for example an ALC(automatic load change), or a multivariable control unit, for example anMPC (model predictive control). In the context of the invention it isadvantageous to use such a system for improving the operating behaviourof the system when the internal compression pressure is varied andthereby to optimize the operating parameters of the distillation columnsystem. The controlled adjustment of these operating parameters ensuresconsistency between the selected internal compression pressure and theoperating point of the distillation and in addition avoids impermissibleloading of the heat exchanger. An essential advantage of use of a loadchange system is the possibility of limiting the gradient of theinternal compression pressure, that is to say the internal compressionpressure does not follow the take-off pressure at any optional speed,but in a controlled manner. This can lead to an increased throttling orto blowing off of the product stream, in the event of rapid change ofthe take-off pressure in a transition phase, even in the processaccording to the invention. In contrast to conventional processes, suchoccurrences proceed only for a short time, however.

The load change system in this embodiment of the invention is constantlyactive and adjusts the preset value for the internal compressionpressure to the current take-off pressure. The preset pressure value ofthe load change system forms the sum of the current take-off pressureand a preselected difference, in order to avoid unnecessary blowing offwhen the take-off pressure rises. Of course, this type of load controlcan be combined with a load change system for the product rates.

In addition, combination with a predictive pressure control of the gaspressure reservoir (for example a pipeline) is advantageous, as isdescribed in EP 1542102 Al. In this case the pressure course in the gaspressure reservoir is determined on the basis of available informationon the future need of the connected end consumers. This can be used inthe context of the present invention for determining the preset pressurevalue for the load change system in order to avoid blowing off productas far as possible.

In a further embodiment of the invention, the elevated pressure (PIV) isonly just above the instantaneous pressure (PA) of the gas pressurereservoir (19); in particular, the difference (PIV−PA) between these twopressures is constantly less than half, in particular less than onethird, in particular less than one fifth, of the range of variation ofthe pressure of the gas pressure reservoir (19). The range of variationof the pressure of the gas pressure reservoir is taken to mean thedifference between the maximum permissible pressure and the minimumpermissible pressure of the gas pressure reservoir.

The invention moreover relates to an apparatus for generating apressurized product by low-temperature air fractionation comprising:

-   -   a distillation column system for nitrogen-oxygen separation,    -   means for feeding compressed, purified and cooled feed air into        the distillation column system for nitrogen-oxygen separation,    -   means for taking off a liquid product stream from the        distillation column system for nitrogen-oxygen separation,    -   means for bringing the product stream in the liquid state to an        elevated pressure (PIV),    -   means for vaporizing or pseudovaporizing the product stream at        the elevated pressure (PIV),    -   means for feeding the (pseudo)vaporized product stream as        pressurized product to a gas pressure reservoir (19),    -   means for varying the elevated pressure (PIV), and    -   a closed-loop or open-loop control unit which varies the        elevated pressure (PIV) as a function of the pressure (PA) of        the gas pressure reservoir.

BRIEF DESCRIPTION OF THE DRAWINGS

Various other features and attendant advantages of the present inventionwill be more fully appreciated as the same becomes better understoodwhen considered in conjunction with the accompanying drawings, in whichlike reference characters designate the same or similar parts throughoutthe several views, and wherein:

FIG. 1 shows a roughly simplified plan of the process and the apparatusaccording to the example; and

FIG. 2 shows a diagram of the time course of the take-off pressure andthe internal compression pressure.

DETAILED DESCRIPTION OF THE DRAWINGS

Air 1 is brought to a first pressure P1 in a main air compressor 2. Thecompressed air 3 is purified in a purification unit 4. The purified air5 is branched into a first substream 6 and a second substream 7. Thefirst air substream 6 is cooled in a main heat exchanger 9 to about dewpoint and flows via the lines 10 and 11 into the distillation columnsystem 12 for nitrogen-oxygen separation which, in the example, has ahigh-pressure column, for example, at 5-15 bar, and a low-pressurecolumn, for example, at 1.2-3 bar, which are in a heat-exchangerelationship via a shared condenser-evaporator, called the maincondenser. The air 11 is introduced into the high-pressure column in avirtually completely gaseous state.

In the distillation column system for nitrogen-oxygen separation 12, theair is fractionated into at least one oxygen-enriched product steam 13and at least one nitrogen-enriched fraction (which is not shown). Theproduct stream 13 has, for example, an oxygen content of 98 to 99.5 mol%. It is taken off in the liquid state, for example from the bottom ofthe low-pressure column or the evaporation space of the main condenser.In a pump 14 the liquid product stream 13 is brought to an elevatedpressure PIV which is higher than the operating pressure of thedistillation column from which it was taken off and is, for example 15to 30 bar. The oxygen 15 is passed at the elevated pressure in liquid orsupercritical state to the cold end of the main heat exchanger 9 and inthe main heat exchanger is vaporized or pseudovaporized and warmed toabout ambient temperature. Via an outlet valve 18, the product streamleaves the system as gaseous pressurized product 16, 18 and isintroduced into a gas pressure reservoir 14 which, in the example, isconstructed as a pipeline system. Via the pipeline system 19, thegaseous pressurized oxygen is finally delivered to a number n, which isin principle as many as is desired, of consumers V1 to Vn.

The pipeline system also serves as product buffer. Depending on theinstantaneous take-off rate, the pressure of the gas pressure reservoir(at the point where line 17 joins) in the example can vary between amaximum permissible pressure of 30 bar and a minimum permissiblepressure of 15 bar.

The heat required for the (pseudo)vaporization is supplied by a heatcarrier stream 21 which is also termed internal compression air and is apart of the second air substream 7 which is recompressed in a secondarycompressor 20 to a high pressure PW which is higher than the firstpressure P1. The pressure P1 is, for example, 5-15 bar, and the pressurePW is, for example, 30 to 40 bar. This pressure in substream 21/22 isadjusted via the valve 8 and the guide vanes of the compressor 20. Atthis high pressure the internal compression air 22 flows through themain heat exchanger 9 to the cold end and in so doing is condensed or,at supercritical pressure, is pseudo-condensed, in indirect heatexchange with the (pseudo)-vaporizing oxygen 15. The internalcompression air is expanded via a valve 30 and at 23 enters, in partliquefied state, the distillation column system 12 for nitrogen-oxygenseparation.

Another part 25 of the second air substream 7/21 is passed out of themain heat exchanger at an intermediate temperature as a turbine stream.Its rate relative to the internal compression air is adjusted via theguide vanes of the turbine. The ratio of the rates of the firstsubstream 6 and second substream 7/21 is set via an expansion valve 30in substream 22.

The turbine air 25 is expanded to about the operating pressure of thehigh-pressure column in an expansion turbine 26. The expanded turbineair 27 is introduced together with the first substream 10 via line 11into the high-pressure column of the distillation column system fornitrogen-oxygen separation 12. The turbine 26 in the example is apreferred element in a cold generation system of this unit, but othertypes of cold generation systems not requiring such a turbine, could beused.

In a conventional manner, the entire air fractionation unit wouldoperate in a steady state and the pump 14 would continuously generate apressure of somewhat more than the maximum take-off pressure of, forexample, 30 bar. The adjustment to the current take-off pressure wouldbe achieved solely by an appropriate throttling in valve 18. Even in thecase of a varying product rate, in pump 14 only the rate of liquidproduct stream 13/15 would be set, but the pressure would remainconstant.

With the invention, in contrast, the outlet pressure of pump 14 isadjusted to the instantaneous take-off pressure. The pump 14 is set toan outlet pressure, or elevated (PIV) pressure, which is about 0.5 to 2bar above the instantaneous take-off pressure, or pressure (PA) of thegas pressure reservoir (14). A certain difference as a margin is logicalin order that, even when the take-off pressure increases, the gaseouspressurized product 16 need not be blown off immediately via line 28 andvalve 29. The corresponding fine adjustment is performed by valve 18 inwhich only a slight pressure decrease is performed, however.

Preferably, not only the stream rates but also the various pressures inthe air fractionation unit, including the parameters of the separationprocess in the interior of the distillation column system 12 fornitrogen-oxygen separation are controlled by means of a central processcontrol system (which is not shown) which is conducted by an automaticload change system. In this case, inter alia the valves 8 and 30 areactivated which determine the rate and pressure of the internalcompression air 22, the valve 24 for establishing the rate of theturbine air 25, the pump 14 for establishing the current rate of theoxygen product and valve 18 for fine adjustment of the product pressureto the take-off pressure. For the exceptional case that it is notpossible to have the unit follow an increasing take-off pressure rapidlyenough, the process control system can also intermittently close valve18 and blow off the gaseous pressurized product into the atmosphere viathe line 28 and the valve 29.

FIG. 2, in the upper part, shows an example of a time course of thetake-off pressure PA and the internal compression pressure PIVqualitatively over a period of five hours plotted along the x axis.

The lower part of the diagram of FIG. 2 is the time course of the ratewhich is delivered by the gas pressure reservoir to the consumers(continuous line).

In the upper part of the diagram, a continuous line shows the course ofthe take-off pressure PA in the pressure reservoir or in the productpipeline of the gas pressure reservoir (the “pressure of the gaspressure reservoir”). The take-off pressure PA can range within therange of variation of the gas pressure reservoir pressure between aminimum operating pressure (min) and a maximum operating pressure (max).When the take-off rate increases (continuous line below) the take-offpressure PA falls (continuous line at top) and vice versa. The internalcompression pressure PIV (the “elevated pressure”) shown as a dashedline at the top follows the course of the take-off pressure PA inprinciple at some distance and with delay. The difference PIV−PA is lessthan one third of the range of variation of the pressure of the gaspressure reservoir.

The internal compression pressure PIV cannot be changed as rapidly asdesired, so that short-term blow off of product can also occur with theprocess according to the invention (see dashed line at the bottom inFIG. 2). The blow-off rate can be kept low, however, by the invention.

In a specific numerical example for delivery of pressurized oxygen to asteelworks, the minimum operating pressure (min) is 20 bar and themaximum operating pressure is 35 bar, the difference PIV−PA is below 2bar, preferably in the range between 0.5 and 1 bar.

Of course, the invention may be applied to any other internalcompression process, in particular to those having differing coldgeneration having one or more turbines, which blow air into thehigh-pressure column and/or the low-pressure column or expand anitrogen-enriched fraction from one of the separation columns of thedistillation column system 12.

The closed-loop control according to the invention can be furtherrefined by evaluating information on the future consumption rates of theconsumers V1 to Vn and obtaining therefrom a prediction of future valuesof the take-off pressure, for example according to the method describedin EP 1542102 A1. The load change system can then move early the stateof the air fractionation unit in a direction which corresponds to theinternal compression pressure PIV required in the future. In thismanner, a still better adjustment of the course of the internalcompression pressure to the take-off pressure can be achieved, whichcontributes significantly to avoiding occasional blow-off of product.

The entire disclosures of all applications, patents and publications,cited herein and of corresponding European application No. EP06007760.9, filed Apr. 13, 2006, are incorporated by reference herein.

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description, utilize the present invention toits fullest extent. The preceding preferred specific embodiments are,therefore, to be construed as merely illustrative, and not limitative ofthe remainder of the disclosure in any way whatsoever.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

1. A process for generating a pressurized product by low-temperature airfractionation, said process comprising: compressing (2), purifying (4),and cooling (9) feed air (1) and then feeding the feed air to adistillation column system (12) for nitrogen-oxygen separation (11, 23),removing a liquid product stream (13) from said distillation columnsystem (12) for nitrogen-oxygen separation, bringing (14) the liquidproduct stream (13), in the liquid state, to an elevated pressure (PIV),and at this elevated pressure (PIV) vaporizing or pseudovaporizing (9)the liquid product stream, feeding (17) the vaporized or pseudovaporizedproduct stream (16) as pressurized product to a gas pressure reservoir(19) which has a variable pressure (PA), varying the elevated pressure(PIV), and varying the elevated pressure (PIV) as a function of thepressure (PA) of the gas pressure reservoir (19).
 2. A process accordingto claim 1, further comprising compressing (20) a gaseous heat carrierstream (7, 21, 22) to a high pressure (PW), and vaporization orpseudovaporization of said liquid product stream (13, 15) is performedby indirect heat exchange (9) with said heat carrier stream which is atthe high pressure, wherein the high pressure (PW) and/or the rate (MW)of the heat carrier stream is varied and the high pressure (PW) and/orthe rate (MW) of the heat carrier stream is varied as a function of thepressure (PA) of the gas pressure reservoir (19).
 3. A process accordingto claim 2, wherein said gaseous heat carrier stream (7, 21, 22) is anair stream.
 4. A process according to claim 1, further comprisingcompressing (20) a gaseous heat carrier stream (7, 21, 22) to a highpressure (PW), and vaporization or pseudovaporization of said liquidproduct stream (13, 15) is performed by indirect heat exchange (9) withsaid heat carrier stream which is at the high pressure, wherein the highpressure (PW) and/or the rate (MW) of the heat carrier stream is variedand the high pressure (PW) and/or the rate (MW) of the heat carrierstream is varied as a function of the elevated pressure (PIV) of theliquid product stream (13).
 5. A process according to claim 4, whereinsaid gaseous heat carrier stream (7, 21, 22) is an air stream.
 6. Aprocess according to claim 3, wherein said gaseous heat carrier streamis a compressed substream of the feed air which after being subjected toheat exchange with said liquid product stream is introduced into thedistillation column system for nitrogen-oxygen separation.
 7. A processaccording to claim 1, wherein cold for the process is obtained in a coldgeneration system (26) and the amount of cold generated in the coldgeneration system (26) is varied as a function of the pressure (PA) ofthe gas pressure reservoir (19).
 8. A process according to claim 7,wherein said cold generation system comprises a substream of the feedair which is subjected to heat exhange with said liquid product stream,expanded, and then introduced into the distillation column system fornitrogen-oxygen separation.
 9. A process according to claim 1, whereinthe operating parameters of the distillation column system are varied asa function of the pressure (PA) of the gas pressure reservoir (19). 10.A process according to claim 1, wherein the elevated pressure (PIV) isonly just above the pressure (PA) of the gas pressure reservoir (19).11. A process according to claim 10, wherein the difference (PIV−PA)between the elevated pressure (PIV) and the pressure (PA) of the gaspressure reservoir (19) is less than half the range of variation of thepressure of the gas pressure reservoir (19).
 12. A process according toclaim 10, wherein the difference (PIV−PA) between the elevated pressure(PIV) and the pressure (PA) of the gas pressure reservoir (19) is lessthan one third the range of variation of the pressure of the gaspressure reservoir (19).
 13. A process according to claim 10, whereinthe difference (PIV−PA) between the elevated pressure (PIV) and thepressure (PA) of the gas pressure reservoir (19) is less than one fifththe range of variation of the pressure of the gas pressure reservoir(19).
 14. A process according to claim 1, wherein an air stream (7, 21,22) is compressed (20) to a high pressure (PW) and the liquid productstream (13, 15) is vaporized or pseudovaporized by indirect heatexchange (9) with said air stream at high pressure.
 15. A processaccording to claim 1, wherein said the distillation column systemcomprises a high-pressure column and a low-pressure column which are ina heat-exchange relationship via a shared condenser-evaporator.
 16. Aprocess according to claim 1, wherein the elevated pressure (PIV) is setto a pressure which is 0.5 to 2 bar above the current the pressure (PA)of the gas pressure reservoir (19).
 17. An apparatus for generating apressurized product by low-temperature air fractionation, said apparatuscomprising: a distillation column system for nitrogen-oxygen separation(12), means (1, 3, 5, 6, 7, 10, 11, 21, 22, 23, 25, 27) for feedingcompressed, purified and cooled feed air into said distillation columnsystem (12) for nitrogen-oxygen separation, means (13, 15) for takingoff a liquid product stream from said distillation column system (12)for nitrogen-oxygen separation, means (14) for bringing said liquidproduct stream in the liquid state to an elevated pressure (PIV), means(9) for vaporizing or pseudovaporizing said liquid product stream at theelevated pressure (PIV), means (16, 17) for feeding the vaporized orpseudovaporized product stream as pressurized product to a gas pressurereservoir (19), means for varying said elevated pressure (PIV) and aclosed-loop or open-loop control unit which varies said elevatedpressure (PIV) as a function of the pressure (PA) of the gas pressurereservoir (19).
 18. An apparatus according to claim 17, wherein said thedistillation column system comprises a high-pressure column and alow-pressure column which are in a heat-exchange relationship via ashared condenser-evaporator.